This invention relates to a method for displaying a figure on the screen of a cathode ray tube (CRT) for use in a numerical control (NC) system. More particularly, the invention relates to a method of displaying a figure on the screen of the CRT of the NC system so that an operator can easily confirm whether or not the figure (a machining locus), which is formed by a machining path of a machining tool (that is, a wire electrode in a wire cut electric discharge machining apparatus) during an operation of machining a workpiece to be machined according to an NC program, includes a closed loop machining path.
FIG. 1 is a block diagram for showing the construction of an NC system used for a conventional wire cut electric discharge machining apparatus (hereunder abbreviated as WEDM) apparatus. In this figure, reference numeral 1 designates a central processing unit (CPU) for processing various data received from other input units; 2, a memory for storing various data; 3, a paper tape input-output (I/O) unit for punching paper tape and reading information recorded in punched paper tape; 4, a CRT for displaying various information; 5, a keyboard for inputting data, instructions, commands and so on; 6, an I/O unit; 7, a flexible disk for receiving and storing data through the I/O unit 6 and outputting therethrough; 8, a data bus for transmitting the data; and 9 a WEDM apparatus for machining a workpiece according to the NC program.
Next, an operation of this conventional NC system will be described hereinbelow.
First, an NC program and data input from the paper tape I/O unit 3, the keyboard 5 and the flexible disk 7 through the I/O unit 6, are analyzed in the CPU 1 so that data representing a machining locus of a wire electrode of the WEDM apparatus on the surface of the workpiece are obtained. Thereafter, the data representing the machining locus are subjected to a wire radius modification processing to obtain actual NC data for driving and controlling the wire electrode of the WEDM apparatus. In the wire radius modification processing, the actual NC data which represents an actual locus of the center of a wire electrode during the machining operation, is obtained by adding both the radius of the wire electrode and a discharge gap to the NC data representing the machined configuration of the workpiece. The actual NC data are then transmitted to the WEDM apparatus 9 through the data bus 8. Further, the CPU 1 can control the CRT 4 to display a graphic form of the machining locus as shown in FIG. 2 on the basis of the NC data transmitted to the WEDM apparatus 9. FIG. 3 is a schematic diagram showing an enlarged view of details of a portion P.sub.A in FIG. 2. In FIG. 3, a wire path 100 (A-B-C-D) indicates the configuration finally machined according to the NC programs. Further, as described above, it is usual in the wire cut electric discharge machining operation to modify the wire path according to the wire radius modification processing to obtain the actual NC data representing the machining locus of the wire electrode by way of shifting the wire path by a sum of the wire radius and the discharge gap. The machining locus modified in this way is shown by a dashed line 101 and 102 (A'-B'-C'-D') in FIG. 3. As is apparent from this figure, in the case where the original wire path (A-B-C-D) is modified with a wire radius modification h larger than a radius r of an arc portion B-C, there occurs a closed loop 102 (E'-B'-C') in the modified machining locus 101 (A'-B'-C'-D'). This closed loop 102 causes an excessive cutting or uncutting to occur in the machined workpiece, as a result of which the configuration of the thus machined workpiece is different from the desired configuration of the workpiece machined according to the NC programmed data, that is the instructed machining configuration. This results in occurrence of defects or other trouble in the EDM operation. It should be noted that it is difficult to detect the occurrence of such a closed loop in the modified machining locus from the graphic display shown in FIG. 2.
In the conventional NC system, the wire path is displayed on the screen of the CRT in such a manner as described above. Hence, the conventional method for displaying the wire path by using the CRT of the NC system is disadvantageous in that it is difficult to readily detect whether or not there occurs a closed loop in the machining locus which may cause a deformation in the workpiece machined according to the actual NC data after the wire radius modification processing.